The present invention relates to a power control apparatus which corrects or compensates displacement angles of phase in AC power systems.
In recent years problems have arisen with respect to higher harmonic currents and/or reactive power which is induced in an AC power line by an electric arc furnace in steel production or by a large-scale power converter in the industry. When a load of the AC line induces much reactive power, power supply equipment should inevitably handle such reactive power as well as active power of the load. Further, a fluctuation of the reactive power provides a voltage variation in the AC line, resulting in unfavorable influence to other electrical equipment. In addition to this, an electromagnetic field induced by the higher harmonic currents adversely interfare with communication lines. Thus, a countermeasure for the higher harmonic currents and controlled generation of reactive power (compensating current) for improving the power factor of the AC line have become of major importance.
There has been proposed an active filter apparatus for reducing said higher harmonic currents and a reactive power compensation apparatus for cancelling the fluctuation of said reactive power.
FIG. 1 shows a conventional configuration of a power control apparatus. In the figure the reference symbol TR denotes a power transformer, La denotes an AC reactor, L01 and L02 respectively denote DC reactors, SS-P denotes a 3-phase bridge-connected converter for the positive component of a circulating current of a cycloconverter, SS-N denotes a 3-phase bridge-connected converter for the negative component of the circulating current, CAP denotes a phase advance capacitor, CTC denotes a current transformer, CC denotes a comparator, Gc(S) denotes a control compensator, -1 denotes an inverting amplifier, PH-P and PH-N respectively denote phase controller, and PTG denotes a 3-phase sine wave generator. The positive converter SS-P, negative converter SS-N and DC reactors L01, L02 jointly constitute a current circulating cycloconverter. Phase advance capacitor CAP serves as a high frequency reactive power source. The oscillation frequency of this high frequency reactive power is defined by the generator PTG.
A compensating current Ic supplied to the AC line is controlled as follows.
Compensating current Ic is detected by current transformer CTC. The detected value of current Ic obtained from current transformer CTC is inputted to comparator CC. Comparator CC receives a compensating current instruction Ic*. Comparator CC compares the detected value Ic with the instruction Ic* and provides an error signal .epsilon. being equal to the difference between the compared ones, i.e., .epsilon.=Ic*-Ic. Error signal e is supplied to control compensator Gc(S). The compensation characteristic or transfer function of compensator Gc(S) is determined based on the stability and response characteristcs of the control system. In the compensator Gc(S) the error signal .epsilon. is subjected to a proportional amplification and/or integration amplification, etc,. An output v.alpha. from compensator Gc(S) is supplied to phase controller PH-P. Output v.alpha. is phase-inverted via inverting amplifier -1 and then supplied to phase controller PH-N. Thus, the relation: EQU ".alpha.N=180.degree.-.alpha.P" (1)
is assigned to the triggering phase angle .alpha.P of positive converter SS-P and that angle .alpha.N of negative converter SS-N. Converters SS-P and SS-N are so controlled that the output voltages from SS-P and SS-N are balanced at both center taps of respective DC reactors L01 and L02.
3-phase sine wave generator PTG provides a phase reference signal vr (3-phase sine wave) to phase controllers PH-P and PH-N. The triggering timing of each of converters SS-P and SS-N is determined based on the phase reference signal vr and the phase signal output v.alpha. from compensator Gc(S).
Now description will be given to the function of compensating current Ic in a case where the voltage Vc appearing across phase advance capacitor CAP, or the voltage Vc of the high frequency reactive power source, is established.
An output voltage vP of positive converter SS-P and an output voltage vN of negative converter SS-N may be represented as follows: EQU vP=kv.multidot.Vc cos .alpha.P (2) EQU vN=kv.multidot.Vc cos .alpha.N=vP (3)
where the symbol kv denotes a proportional constant. Then, the output voltage vo of the cycloconverter is: EQU vo=(vP+vN)/2=vP (4)
When the turning ratio of transformer TR is 1:1, the voltage difference (vo-vs) between the cycloconverter output voltage vo and the power source voltage vs is applied to AC reactor La, and the compensating current Ic flows through the reactor La.
When the detected value of compensating current Ic is below the value of instruction Ic*, the error .epsilon.=Ic*-Ic becomes positive, resulting in increasing of the phase signal output v.alpha. and the cycloconverter output voltage vo. The increase of output voltage vo causes to enlarge the voltage difference (vo-vs) applied to AC reactor La. Then, the compensating current Ic becomes large and the detected value of Ic comes close to the value of instruction Ic*.
When the detected value of compensating current Ic exceeds the value of instruction Ic*, the error .epsilon.=Ic*-Ic becomes negative, resulting in decreasing of the phase signal output v.alpha. and the cycloconverter output voltage vo. The decrease of output voltage vo causes to reduce the voltage difference (vo-vs) applied to AC reactor La. Then, the compensating current Ic becomes small and the detected value of Ic comes close to the value of instruction Ic*.
The compensating current instruction Ic* contains information of an active component being in-phase to the power source voltage vs, information of a reactive component whose phase is deviated by 90 degrees from the voltage vs, information of higher harmonics for compensating higher harmonic currents, and so on. The compensating current Ic is controlled such that the actual value of current Ic coincides with said instruction Ic*. When the cycloconverter receives an active component of voltage vs from the AC power source, the voltage Vc of phase advance capacitor CAP rises. When the cycloconverter supplies an active current component to the AC power source, the voltage Vc falls. A certain active current should be fed into the cycloconverter from the AC power source so as to cancel the operation loss of the apparatus, so that the voltage Vc of phase advance capacitor CAP is kept constant. Incidentally, an averaged reactive component of compensating current Ic has no effect to the voltage Vc.
The triggering timing of converters SS-P and SS-N is determined by the signal crossing point between the signal output from PTG and the phase signal v.alpha., and the frequency (fc) of the high frequency reactive power source is determined according to this triggering timing. The oscillation frequency fc of the high frequency reactive power source is equal to the output frequency of 3-phase sine wave generator PTG. That is, the operation of the high frequency reactive power source is similar to that of an externally controlled cycloconverter. In the operation of this reactive power source, a circulating current Io of the cycloconverter flows so that an oscillation condition (or resonance condition) of the cycloconverter is satisfied. At the time of power-on of the power control apparatus, the voltage Vc of phase advance capacitor CAP may be established in a manner that an active current component fed from the AC power line to the cycloconverter is used for the instruction Ic* of compensating current Ic.
FIG. 2 illustrates the relation among the compensating current Ic, circulating current Io of the cycloconverter and oscillation frequency fc of the high frequency reactive power source shown in FIG. 1. Voltage Vc and oscillation frequency fc are controlled to keep them substantially constant and are independent of the magnitude of compensating current Ic. Thus, a phase-advanced reactive current Icap flowing through phase advance capacitor CAP is: EQU Icap=Vc.multidot.2.pi.fc.multidot.C=constant (5)
where C denotes the capacitance (F) of capacitor CAP. The cycloconverter consumes the delayed reactive power from the high frequency power source. The oscillation condition of the high frequency power source is established at a state wherein the delayed reactive power is equal to the advanced reactive power from capacitor CAP. Thus, the oscillation condition is: EQU IQ=Icap (6)
where IQ denoted a delayed reactive current of the delayed reactive power.
Now, consideration is given to the delayed reactive current IQ. Suppose that a triggering phase angle of positive converter SS-P and an output current therefrom are denoted by .alpha.P and IP, respectively, and that a triggering phase angle of negative converter SS-N and an output current therefrom are denoted by .alpha.N and IN, respectively. Then, delayed reactive current IQ may be represented as: EQU IQ=KI(IP.multidot.sin .alpha.P+IN.multidot.sin .alpha.N) (7)
where KI denotes a conversion constant.
FIG. 2A shows waveforms of the output currents IP and IN. In this figure, the dashed line is a waveform of the compensating current Ic corresponding to IP+IN, and the DC shift level of each of currents IP and IN corresponds to the circulating current Io. Based on the consideration of Eq. (1) and FIG. 2A, Eq. (7) may be modified as: ##EQU1## where .vertline.Ic.vertline. denotes the absolute value of compensating current Ic and Io denotes the circulating current. From Eq. (8), the specific circulating current Io satisfying the oscillation condition of Eq. (6) is represented as: EQU Io=(1/2)[IQ/(KI.multidot.sin .alpha.P)-.vertline.IC.vertline.] (9)
Incidentally, exactly speaking, the amount of circulating current Io transiently varies as the transient change of triggering phase angle .alpha.P and/or compensating current Ic. However, FIG. 2 illustrates the average of circulating current Io.
According to the conventional power control apparatus as mentioned above, when the magnitude of compensating current Ic exceeds a certain value, the absolute value .vertline.Ic.vertline. of compensating current Ic becomes larger than the term "IQ/(KI.multidot.sin .alpha.P)" of Eq. (9). In this case, so long as the oscillation condition of Eq. (6) is established, the polarity of circulating current Io must be negative, but a negative circulating current cannot be obtained. From this, the maximum value of compensating current Ic of the conventional apparatus is restricted to the value being obtained at Io=0. Thus, the maximum compensating current Icmax of the conventional apparatus is: EQU Icmax=IQ/(KI.multidot.sin .alpha.P)=Icap/(KI.multidot.sin .alpha.P) (10)
Eqs. (9) and (10) teach that, when large Icmax is required in a certain application of the power control apparatus, the amount of circulating current Io at Ic=0 becomes inevitably large, resulting in increasing of unnecessary power loss due to the large amount of circulating current Io.